The genus Thymus (Lamiaceae) is represented by 38 species (64 taxa) in Turkey, and 24 of which are endemic to Turkey. Aerial parts of Thymus longicaulis subsp. chaubardii var. chaubardii, T. zygioides var. lycaonicus, T. longicaulis subsp. longicaulis var. subisophyllus and T. pulvinatus collected from three different localities in Balikesir province were subjected to hydrodistillation to yield essential oils which were subsequently analysed by GC and GC/ MS. The main constituents of the oils were identified, and antimicrobial bioassay was applied. Thymol (56.6%, 42.8%, 36.9%) was the main component in the oils of T. longicaulis subsp. chaubardii var. chaubardii (chemotype I), T. longicaulis subsp. chaubardii var. chaubardii (chemotype II) and T. zygioides var. lycaonicus respectively. The oil of T. longicaulis subsp. longicaulis var. subisophyllus contained carvacrol (60.0%) and the oil of T. pulvinatus borneol (27.9%) as main constituents.